Erbium-doped fiber amplifier and integrated circuit module components

ABSTRACT

An EDFA with integrated input and output modules is presented. The integrated input module has a packaged pump laser diode mounted to the metal EDFA package to provide a heat sink for the pump laser diode which sends the pump laser light over a optical fiber section connected to the amplifying erbium-doped optical fiber section. The fiber section is formed from an optical fiber which better matches the transmission modes in the erbium-doped optical fiber section and has an end subsection of the single mode fiber for a larger numerical aperture. Collimating lenses also increase the coupling efficiency of the laser diode to the erbium-doped fiber section. The integrated output module has a photodiode with a tap filter to monitor the output power of the EDFA, an optical isolator to prevent interference in the erbium-doped optical fiber section. With a twin optical isolator, the integrated input and output modules can be arranged in different ways and combinations with the erbium-doped optical fiber section. The resulting EDFAs can be manufactured relatively inexpensively into an very small packages compared to current EDFA packages.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/138,033 filed May 3, 2002 now U.S. Pat. No. 6,922,281, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is related to optical devices for fiberopticsystems and networks and, in particular, to erbium-doped fiberamplifiers and their components.

In modern fiberoptic transmission systems and networks, such as DenseWavelength Division Multiplexing (DWDM) optical networks, theerbium-doped fiber amplifier (EDFA) is employed nearly universally toamplify optical signals. The EDFA provides for optical-to-opticalconversion and avoids OEO (Optical-Electrical-Optical) conversion wherereceived optical signals were converted into electrical signals,retimed, reshaped and regenerated back into optical signals. EDFAs havethe advantages of wideband, i.e., the ability to amplify signals over awide range in frequency, high signal gain, low noise figure, high outputpower and low polarization sensitivity.

Hence the EDFA provides for savings in cost and complexity. Nonetheless,EDFAs today are still expensive and are used primarily in the so-called“long-haul” or “backbone” fiberoptic networks which link nodes overlong, intra-continental and even inter-continental, distances. Most ofthese fibers have been laid and the present challenge to fiberopticdevelopers are the “metro-,” i.e., citywide, or smaller, networks. Muchdevelopment effort has been directed toward a compact (for ease ofinstallation) and cost-effective optical amplifier for metro-networksand upgrades of optical nodes in the long-haul networks. This effortincludes conventional wideband EDFAs, semiconductor optical amplifiers(SOAs) and erbium-doped waveguide amplifiers (EDWAs). However, widebandEDFA is quite expensive for some applications, such as metro-networksand power compensation. In this case, narrow band EDFA is much more costeffective than wideband counterparts. SOAs have disadvantages of highnoise figures, polarization-dependent gain (PDG) and cross talk; theirapplications are very limited. Likewise, EDWAs require very high pumppower to provide sufficient gain and output power, while their noisefigures are quite high. Therefore, EDFAs are still the most efficientapproach in power conversion efficiency.

The present invention provides for a novel, low-cost, and highly compactEDFA.

SUMMARY OF THE INVENTION

The present invention provides for an EDFA system for amplifying opticalsignals received from one optical fiber and passing the amplifiedoptical signals to the other optical fiber. The EDFA system has asection of erbium-doped optical fiber with each of its ends coupled toone of the optical fibers; and at least one integrated input modulehaving a first optical fiber section connected to an end of theerbium-doped optical fiber section and a second optical fiber sectionconnected to one of the optical fibers. The integrated input module hasa WDM filter arranged with respect to the ends of the optical fibersections so that optical signals received from one of the optical fibersections is passed to the other of the optical fiber sections. Theintegrated input module has a laser diode arranged with respect to theWDM filter and the ends of the optical fiber sections so that pump lightfrom the laser diode is passed to the first optical fiber section andthe erbium-doped optical fiber section. The laser diode is mountedwithin an TO package in the integrated input module and the laser diodepackage is thermally connected to a metal package for the EDFA system toprovide a heat sink for the laser diode operating a pump laser for theerbium-doped optical fiber section.

To increase the coupling efficiency of the laser diode to theerbium-doped optical fiber section, the integrated input module has twocollimating lens for focusing the pump laser light on the end of firstoptical fiber section. The first optical fiber section is formed from afiber which matches the transmission modes of the erbium-doped opticalfiber section, but the end of the first fiber section is formed from asingle mode fiber, SMF-28, for a larger numerical aperture.

Likewise, the integrated output module has many feature to efficientlymonitor the output of the erbium-doped optical fiber section and tooptically isolate the erbium-doped optical fiber section from errantsignals. With a twin optical isolator, various combinations of theerbium-doped optical fiber section, integrated input module andintegrated output module can be made.

An EDFA system can be manufactured relatively inexpensively and mountedin a very small package compared to current EDFAs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representational diagram of a conventional EDFA;

FIG. 2 is a representational diagram of a conventional EDFA withadditional components; FIG. 2B is a representational diagram of aconventional EDFA with an integrated tap/isolator component; FIG. 2C isa representational diagram of a conventional EDFA with an integratedcomponent; FIG. 2D is a representational diagram of a conventional EDFAwith an integrated pump laser component;

FIG. 3 is a representational diagram of an EDFA system according to oneembodiment of the present invention;

FIG. 4A is a representational drawing of an integrated input module ofthe FIG. 3 EDFA; FIG. 4B is a representational drawing of a variation ofthe integrated input module of the FIG. 4A;

FIG. 5A is a detailed side view of the sleeve holding the ends of thefiber sections of the integrated input module of FIGS. 4A and 4B; FIG.5B is a detailed side view of the end of the output fiber section heldin the FIG. 5A sleeve;

FIG. 6 is a representational drawing of an integrated output module ofthe FIG. 3 EDFA;

FIG. 7 illustrates the operation of the optical isolator in theintegrated output module of FIG. 6 with the cross-sectional front viewsof the different elements of the optical isolator;

FIG. 8A is a block diagram of a double-pumped EDFA system with a twinoptical isolator, according to another embodiment of the presentinvention; FIG. 8B is a cross-sectional diagram of the twin opticalisolator in the FIG. 8A EDFA system; and

FIG. 9 is a block diagram of an EDFA system with a twin optical isolatorand no integrated output module, according to still another embodimentof the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

To amplify optical signals carried on an optical fiber, the opticalfiber is severed and an EDFA is inserted between the two parts of theoptical fiber. Optical signals are typically in the 1550 nm wavelengthrange; for WDM systems, the signals fall within specific wavelengthranges which define a grid of WDM communication channels. The EDFAincreases the strength of these signals at their particular wavelengths.

FIG. 1 illustrates the general features, organization and operation ofan EDFA between two parts 18 and 19 of an optical fiber carryingsignals. The EDFA is formed by a section 10 of erbium-doped fiber, whichamplifies the optical signals passing through the section 10. To providethe amplification energy for the EDF section 10, an optical pump in theform of a laser 11 is connected to one end of the section 10 by a WDM(Wavelength Division Multiplexer) 12. The WDM 12 is connected to theinput optical fiber 18 which carries the optical signals into the EDFAand an output fiber from the pump laser 11. To block signals in the“wrong” direction and to monitor the operation of the EDFA, an opticalisolator 13 and a tap coupler 14 are inserted between the other end ofthe EDF section 10 and the output optical fiber 19. The tap coupler 14sends a small fraction of the signals from the section 10 to aphotodiode 15. In the drawings fiber splices are indicated by thesymbol, “X”.

Operationally, the pump laser 11 generates light at energies higher thanthose of the communication signals in the 1550 nm range. Typically, theoutput of the pump laser 11 is at 980 nm, but other wavelengths arepossible and may be used. The WDM 12 combines signals from the inputoptical fiber 18 and light of the pump laser 11 and sends both into theEDF section 10 which amplifies the 1550 nm wavelength signals from theenergy provided by the 980 nm light. The amplified signals are passed tothe output optical fiber 19 with the isolator 13 preventing signals fromthe output optical fiber 19 from entering the EDF section 10 and causinginterference. The EDFA operation is monitored by the photodiode 15 andmay be used to provide feedback control of the EDFA, such as forautopower control or autogain control. Likewise, the pump laser 11 oftenhas a rear photodiode (PD) which monitors the output power of a laserdiode (LD), the lasing element of the pump laser 11. In other EDFAarrangements, two pump lasers are often used for double pumping of theEDF section.

The drawbacks for this configuration is that there are a large number ofdevices, splicing losses between the SMF-28 optical fiber (e.g., theinput optical fiber 18 and output optical fiber 19 of FIG. 1) which isconventionally used to the optical signals and the erbium-doped fibersection, potential noise figure degradation due to the insertion lossfrom the different single devices and the splicing points. In fact, inmost EDFA applications, more optical components are added to the EDFA.Such arrangements are illustrated below.

In the EDFA arrangement shown in FIG. 2A, an optical isolator is added.This arrangement prevents back reflection to optical signal source andprovides the function of optical power monitoring. However, morecomponents imply more insertion loss and splicing loss to the opticalsignals. To improve EDFA performance and to decrease the size of thepackage, hybrid integrated devices have been developed. FIGS. 2B–2D showdifferent configurations with hybrid integrated devices.

FIG. 2B illustrates an EDFA with a combined device to decrease thenumber of splicing points and to improve reliability. In this example,the hybrid integrated device includes a pump rejection filter, a tapcoupler filter and optical isolator. In FIG. 2C the hybrid integrateddevice is an IFAM (Integrated Fiber Amplifier Module), which includes apump/signal WDM, a tap filter photodiode and optical isolator. In themeantime, an extra pump rejection filter removes the residual pump powerat the input port of the EDFA. For the EDFAs of FIGS. 2B and 2C, thesplicing of the single mode fiber with EDF section increases the noisefigure and back reflection.

FIG. 2D illustrated an EDFA with an improved IFAM. The pump laser diodechip is added to the hybrid integration module. Although integration isincreased, there are yield and reliability problems with the addedcomplexity of the module. A so-called “butterfly” package must be used,instead of a more desirable cylindrical package, resulting in a largepackage. Higher cost is also a result of the FIG. 2D arrangement.

All these present day EDFA systems have certain common disadvantages.The various component devices are separate or integrated incomparatively large and expensive packages. The pump laser diode whetherin a separate package or integrated with other elements must be cooledwith the corresponding control circuit complexity and cost. Finally, thesplice connection to the amplifying erbium-doped fiber section is madewith a mode-mismatched single mode optical fiber, the standard singlemode fiber being SMF-28 from Corning, Inc. of Corning, N.Y. A highcomponent count requires more fiber coupling and fusion sections on bothsignal and pumping paths; these undesirably affect the signal noisefigure and pumping efficiency. Furthermore, more assembly of the EDFAcomponents add more costs and lowers the reliability of the assembledsystem.

To ameliorate these problems, the present invention provides for an EDFAsystem, such as illustrated in FIG. 3, according to one embodiment ofthe present invention. The EDFA system has an erbium-doped fiber section20, an integrated input module 21 and an integrated output module 22.The integrated input module 21 receives a optical signals on an inputoptical fiber 28, generates pump light, and sends the combined pumplight and signals to the EDF section 20, while an integrated outputmodule 22 which transmits the signals from the EDF section 20 to anoutput optical fiber 29 while monitoring the strength of the amplifiedoptical signals. The integrated output module 22 also blocks signals inthe reverse direction from the output optical fiber 29 to the EDFsection 20 and also blocks residual pump power light from the EDFsection 20 to the optical fiber 29. The input and output fiber sections28A and 24 respectively of the integrated input module 21 are formedfrom different optical fibers. The input fiber section 28A is formedfrom a single mode fiber to match the input optical fiber 28 to whichthe input fiber section 28A is spliced. This reduces the insertion lossof the splice. Likewise, the output fiber section 24 is formed from anHI-980 optical fiber, which better matches the EDF section 20 than asingle mode fiber.

The organization and elements of the integrated input module 21 areillustrated in FIG. 4A. The integrated input module 21 has in the formof an packaged pump laser diode 34. The package is a TO can,specifically, an uncooled CD-5.6 TO package. In front of the packagedpump laser diode 34 is a laser diode collimating lens 33, a WDM filter32 which discriminates between 1550 nm wavelength signals and 980 nmwavelength light, i.e., between message signals and pump light, acollimating GRIN lens 31, and a glass sleeve 30 with a central capillarychannel holding ends of the input fiber section 28A and output fibersection 24. To reduce unwanted reflection, the facets of the sleeve 30and GRIN lens 31 are reciprocally angled by a small amount,approximately 8°. As described in detail below, the output fiber section24 also has a fiber collimator and mode transition with a modetransition subsection. All of the elements of the integrated inputmodule 21 are physically symmetrical about a central axis so that theelements readily fit into a cylindrical package, as symbolicallyindicated by the enclosing dotted line.

The uncooled TO packaged pump laser diode 34 also has a rear photodiode(not shown) for monitoring the output of the laser diode. In contrast tothe requirements of handling and mounting a semiconductor die, thealready packaged diode 34 saves costs in manufacturing and reliability.CD-5.6 TO package is a well-known, low-cost package format for CD laserand low-end telecom lasers and is preferred in the integrated inputmodule 21. With proper heat dissipation design, the metal package forthe entire EDFA system is used as a heat sink for the TO package of thepump laser diode. This allows a CD-5.6 TO packaged laser diode to beeffectively driven at high power and the typical manufacturing costs fora EDFA pump laser is significantly lowered. Such packaged laser diodesare available from several suppliers, such as Mitsubishi ElectronicCorporation of Tokyo, Japan; Sharp Electronics Corporation, of Osaka,Japan; and Blue Sky Research of Milpitas, Calif.

The flat window 35 in the package of the pump laser diode 34 is coatedwith antireflection material to prevent facet damage at high opticalpower, while providing the possibility of wavelength stabilization witha fiber Bragg grating (FBG) placed in the output fiber 24. Because thedistance between the active layer of the pump laser diode and the flatwindow 35 of the TO can package is around 0.5 mm, a laser diodecollimating lens 33 (different lenses may be used, including asphericallens, and flat or spherical facet GRIN lens, as shown in embodiments ofthe integrated input module 21 in FIGS. 4A and 4B) to help collimate theoutput light from the pump laser diode 34 onto the GRIN lens 31. Theflat surface of the GRIN lens 31 has deposited coatings of thin filmdielectric materials to form the WDM filter 32 reflect the 1550 nmsignal light and transmits the 980 nm pump light. Accurate control ofthe L₁, the distance between the window 35 and the laser diodecollimating lens 33, and L₂, the distance between the laser diodecollimating lens 33 and the WDM filter 32 at the end of the collimatingGRIN lens 31, is important for improving the coupling efficiency of thepump light into the output fiber section 24. In the describedembodiment, L₂ is usually less than 0.7 mm and L₁ is approximately 2 mmdepending upon the WDM filter 32 and the length accuracy of the GRINlens 31.

Besides the focusing action of the laser diode collimating lens 33 toimprove the coupling efficiency, the integrated input module 21 has amode-expanded and transition fiber subsections to improve the opticalpower coupling from the pump laser diode to the HI-980 fiber in theoutput fiber section 24. FIG. 5A is an expanded cross-sectional view ofthe glass sleeve 30. In its central capillary channel are the ends ofthe input fiber section 28A and output fiber section 24. The two fibersections 28A and 24 are in the same capillary and are drawn separated toillustrate that there are two optical fibers. The end of the input fibersection 28A is a simply a single mode optical fiber, a SMF-28 fiber. Onthe other hand, the end of the output fiber section 24, enclosed by anellipse 40 is not HI-980 optical fiber.

FIG. 5B is a detail of the output fiber section 24 enclosed by theellipse 40. Most of the output fiber section 24 is indeed HI-980 opticalfiber, which forms the so-called “fiber pigtail,” the free optical fiberwhich is to be connected to another optical fiber, in this case, the endof the EDF section 20. At the end of HI-980 fiber indicated by thenumeral 24A, is a fiber transition subsection 39 and the mode-expandedsubsection 38. The subsections 38 and 39 are formed by fusing an end ofSMF-28 optical fiber to the end of the HI-980 fiber. The SMF-28 fiber iscut and the attached single mode fiber forms the mode-expandedsubsection 38 and the fused portions of the SMF-28 and HI-980 fibersform the fiber transition subsection 39. It has been found that a totallength of 1.0–1.5 mm for the mode-expanded subsection 38 and fibertransition subsection 39 respectively work effectively. With thesesubsections 38 and 39, the reflection loss for the incoming opticalsignals at 1550 nm on the input fiber section 28A and reflected by theWDM filter 32 is decreased considerably. From the FIG. 5B drawing, it isevident that the SMF-28 fiber subsection provides a larger numericalaperture than one formed from HI-980 optical fiber. The reducedinsertion loss of the integrated input module 21 improves the noisecharacteristics of the EDFA system.

FIG. 4B illustrates the integrated input module 21 with an asphericalfacet GRIN lens 36 in place of the flat facet GRIN lens 33 of FIG. 4A.Coupling efficiencies of 40% and 50% respectively have been obtained bycombining the mode-expanded transition fiber pigtail with the flat facetGRIN lens 33 of FIG. 4A and of the aspherical facet GRIN lens 36 of FIG.4B.

FIG. 6 shows the elements and organization of the integrated outputmodule 22 of EDFA system according to the present invention. Theintegrated output module 22 has the elements of a reflective opticalisolator, a 3% tap filter, a pump rejection filter and a powermonitoring photodiode. The integrated output module 22 has a glasssleeve 40 with a central channel capillary channel holding ends of aninput fiber section 25 (connected to the EDF section 20) and an outputfiber section 29A (connected to the output fiber 29), a walk-offbirefringent crystal plate 41 mounted at one end of the sleeve 40, acollimating GRIN lens 43 having a zero-order half-wave plate 42 coveringa portion of the GRIN lens end surface facing the sleeve 40, a plate 44of latching garnet for a 22.5° Faraday rotator on the opposite endsurface of the GRIN lens 44, a highly reflecting tap filter 45 of thinfilm materials deposited upon the Faraday rotator plate 44, a WDM filter46 of thin film materials deposited on the tap filter 45, and aphotodiode 48. Due to the walk-off crystal 41 attached directly to theend surface of the sleeve 40, a quarter-pitch GRIN lens is not used forthe GRIN lens 43, but rather a 0.23 pitch GRIN lens or an aspherical rodlens (C-lens) may be employed for collimating light. The elements,except for the half-wave plate 42 of the integrated output module 22 arephysically symmetrical about a central axis so as to readily fit into acylindrical package, as symbolically indicated by the enclosing dottedline. Furthermore, the facing end surfaces of the sleeve 40 and GRINlens 43 are reciprocally angled by a small amount, approximately 8°.

The reflective optical isolator is formed by the walk-off birefringentcrystal 41, such as YVO₄ or rutile, 0.20 mm thick, the zero-orderhalf-wave plate 42, and the Faraday rotator plate 44. The optical axisof the walk-off crystal 41 and the half-wave plate 42 are oriented suchthat the light signals from the output fiber section 29A are blockedfrom being reflected back to the input fiber section 25, while signalsfrom the input fiber section 25 are reflected back to the output fibersection 29A.

The cross-sectional frontal views of FIG. 7 illustrate the operation andfunction of each of the elements which comprise the optical isolator ofthe integrated output module 22. The initially horizontal and verticalbars over the circles representing the output fiber section 29A andinput fiber section 25 are the orthogonal, linear polarization states ofthe light coming into the integrated output module 22. Again forpurposes of explanation, the circles are displaced from each other,though the output fiber section 29A and input fiber section 25 fixedtogether in the capillary channel of the sleeve 40. The dotted arrowsfrom the output fiber section 29A and input fiber section 25 circlesindicate the source of the represented polarization state. Hence theinitial action of the walk-off crystal 41 displaces upward thevertically polarized light from the output fiber section 29A and inputfiber section 25, while the locations of the horizontally polarizedlight are unchanged. The action of the highly reflecting tap filter 45horizontally displaces the horizontally and vertically polarized lightfrom one fiber section toward the other fiber section. The net result isthat the horizontally and vertically polarized light from the inputfiber section 25 is combined at the end of the output fiber section 29A;while the horizontally and vertically polarized light from the outputfiber section 29A is not combined and neither of these polarizationcomponents fall on the end of the input fiber section 25. Light travelsfrom the input fiber section 25 to the output fiber section 29A, but notin the opposite direction.

The input fiber section 25, which is spliced to the EDF section 20, isalso formed from HI-980 optical fiber with a mode expansion and modetransition subsections at the fiber's end, as described with respect toFIG. 5B. Together with a proper orientation of the optical axis of thewalk-off crystal 41 and half-wave plate 42, the mode expansion andtransition subsections obtain low insertion loss and high isolationperformance for the integrated output module 22.

Furthermore, the integrated output module 22 also filters out anyresidual laser pump light from the input fiber section 25 and the EDFsection 20. The latching garnet Faraday rotator plate 44 is highlyabsorbent to 980 nm light. Light at that wavelength suffers a 20 dB lossin passing through the plate. Secondly, the tap filter 45 is highlyreflective and reflects back most of the light to the output fibersection 29A. Nonetheless, the tap filter 45 allows about 3% of theoutput optical power to pass through to the photodiode 47 to monitor theoutput optical power of the EDFA system. The WDM filter 46 is extremelyreflective to 980 nm wavelength light and prevents the residual pumplight from entering the photodiode 48. The leakage of pump laser lightinto the photodiode 48 deteriorates the accuracy of the optical powermonitoring and decreases the dynamic range of the optical power monitor.Since the location of WDM filter 46 is displaced from the tap filter 45with respect to the ends of the fiber sections 25 and 29A, any 980 nmlight reflected by the filter 46 is not focused at the end of the outputfiber section 29A.

The described EDFA system of FIG. 3 can be highly miniaturized. Theintegrated input module 21 can be fitted into a cylindrical package of25 mm length and 6 mm diameter; likewise, the integrated output module22 can be fitted into a cylindrical package of 18 mm length and 4 mmdiameter. With the splice connections to HI-980 optical fiber, the EDFsection 20 can be wound more tightly into loops of 24 mm diameter.Altogether, the complete EDFA system can be fitted into a metal packageof 40 mm×70 mm×12 mm, a very small EDFA system package. Hence theadvantages of the present invention include 1) the integratedinput/output modules are cylindrical to decrease the package size; (2)only HI-980 fiber and erbium-doped fiber are used within the EDFApackage to make it even smaller; (3) the TO can pump laser diode in theintegrated input module significantly reduces the cost of the pump laserdiode; (4) the pump laser is uncooled for a reduction in control circuitcomplexity and in power consumption; and (5) the optical components areshared for additional cost reductions.

FIG. 8A illustrates another EDFA system within a twin optical isolatorto provide additional cost savings, according to the present invention.The EDFA system has an erbium-doped fiber section 20 which isdouble-pumped by two integrated input modules 21A and 21B at either endof the section 20, and a twin optical isolator 23. For the EDFA systemillustrated in FIG. 8A, the input optical fiber is connected to aminiature optical power monitor 50 to check on the power of the incomingoptical signals. The power monitor 50 is connected by an optical fibersection 53 to the twin optical isolator 23 which is connected to a firstintegrated input module 71A. The optical isolator 23 ensures thatoptical signals only travel from the input optical fiber to theintegrated input module 21A where pump laser signals are combined withthe optical signals and passed to the EDF section 20 through aHI-980/EDF splice 76A. The other end of the EDF section 20 is connectedto a second integrated input module 71B by a corresponding EDF/Hi-980splice 76B. The integrated input module 71B sends its pump signals tothe section 20 through its HI-980 fiber section 74B and receives theamplified signals of the section 20 through the same fiber section 74B.The amplified signals are passed through to the output fiber section 78Bwhich is connected to the twin optical isolator 23. The input and outputroles of the fiber sections 74B and 78B of the second integrated inputmodule 71B are reversed compared to the physically corresponding fibersections 74A and 78A of the first integrated input module 71A. Returningto the twin optical isolator 23, the corresponding output of the outputfiber section 78B is a fiber section 54 connected to the output fiber ofthe EDFA system through a second miniature optical power monitor 51which checks on the power of the outgoing optical signals from the EDFAsystem.

Serving two isolation functions, the twin optical isolator 23 savespackage space and cost. FIG. 8B shows the schematic diagram of the twinisolator with a bandpass filter. The optical isolator 23 has a firstglass sleeve 60 with a central capillary channel which holds the ends ofthe optical fiber section 53 and output fiber section 78B of the secondintegrated input module 71B. A collimating lens 61 is located in theinterior end of the sleeve 60. Facing in the opposite direction is asecond glass sleeve 64 with a central capillary channel which holds theends of the input fiber section 78A of the integrated input module 71Aand the fiber section 54. A second collimating lens 63 is located at theinterior end of the sleeve 64 to face the first collimating lens 61. Thecollimating lenses 61 and 63 may be GRIN lens or C-lens, but theC-lenses have better performance and yields than conventional GRINlenses. Between the collimating lenses 61 and 63 is a conventionaloptical isolator core 65 formed by a sandwich structure of birefringentcrystal wedge/Faraday rotator/birefringent crystal wedge. Details ofsuch optical isolators are found in the literature. See, for example,U.S. Pat. No. 5,208,876, which issued May 4, 1993. The ends of the inputfiber sections 53, 78B and the output fiber sections 78A, 54 arearranged and oriented so that light from the section 53 is transmittedto the section 78A and light from the section 78B is transmitted to thesection 54. Of course, light in the opposite direction is blocked.

Deposited on the interior end of the collimating lens 63 are thin filmsforming a bandpass or lowpass filter 66. The bandpass or lowpass filter66 can be employed for flattening the gain and/or for ASE reduction ofthe EDFA system.

FIG. 9 illustrates still another EDFA system with a twin opticalisolator, according to another embodiment of the present invention. TheEDFA system has an erbium-doped fiber section 20, and only oneintegrated input module 71 and a twin optical isolator 73 which does nothave an internal filter. The FIG. 9 EDFA system is similar to the FIG.8A system; however, the output end of the EDF section 20 forms one ofthe input fibers to the twin optical isolator 73. Furthermore, abandpass or lowpass filter 55 is placed at the input end of the twinoptical isolator 73 and is external to the twin optical isolator.Alternatively, a bandpass or lowpass filter may be placed at the outputend of the twin optical isolator 73 as indicated by the dotted rectangle56.

A low-cost, high miniaturized narrowband EDFA has many uses andapplications. The EDFA can be used over the C-band of a WDM network as abooster amplifier to provide power amplification to the opticaltransmitter and as a pre-amplifier for an optical receiver to amplifyweak optical signals. The applications include single channel opticalfiber transmitter systems, WDM transmitter systems, opticalcross-connects and wavelength add/drop multiplexers, metro/edge networksand optical transceiver modules.

In a high-speed single channel system, for example, a high saturatedoutput power booster EDFA for the transmitter and a low noise figurepre-amplifier for the receiver can be combined to reach highSignal-to-Noise ratios (SNRs) and a low Bit Error Rate (EBR). The EDFAcan be tailored for particular system requirements.

The narrowband EDFAs can be used as optical amplification elements forbands of WDM channels in an optical VMUX/DEMUX to decrease the totalcost of optical amplifiers in the network and to increase theflexibility and upgradability of the optical network. (A VMUX is acombined variable optical attenuator (VOA) and multiplexer (MUX) for thefunctions of WDM multiplexing and power balancing in the WDM channels inone device.) For an optical add/drop multiplexer in the network, theEDFA can be used to amplify signals dropped from the network, or signalsbefore they are added to the network. The EDFA can used to preamplifysignals before they are processed by a dispersion compensator module andto amplify the signals after they are processed. In an opticalcross-connect system, EDFAs can amplify signals entering thecross-connect system and leaving the system to compensate for opticalloss in the optical path.

Therefore, while the description above provides a full and completedisclosure of the preferred embodiments of the present invention,various modifications, alternate constructions, and equivalents will beobvious to those with skill in the art. Thus, the scope of the presentinvention is limited solely by the metes and bounds of the appendedclaims.

1. An integrated input module for an EDFA for amplifying opticalsignals, said integrated input module comprising a sleeve having alongitudinal capillary channel holding ends of a first optical fibersection and a second optical fiber section; a laser diode mounted in apackage, said laser diode generating laser pump light though said laserdiode package; a first collimating lens proximate said laser diode; asecond collimating lens proximate said sleeve and said ends of saidfirst optical fiber section and said second optical fiber section, awavelength-dependent filter between said first and second collimatinglens, said wavelength-dependent filter passing laser pump light andreflecting said optical signals, said ends of said optical fibersections, said laser diode, said first collimating lens, said secondcollimating lens, and said wavelength-dependent filter arranged andoriented with respect to each other so that laser pump light from saidlaser diode is focused on said first optical fiber section end and sothat optical signals from one of said optical fiber sections is passedto the other of said optical fiber sections.
 2. The integrated inputmodule of claim 1 wherein said laser diode package comprises a TO-can.3. The integrated input module of claim 1 further comprising acylindrical package holding said sleeve, said laser diode package, saidfirst collimating lens, said second collimating lens, and saidwavelength dependent filter.
 4. The integrated input module of claim 3wherein said cylindrical package has dimensions no larger than 25 mmlong and 6 mm diameter.
 5. The integrated input module of claim 1wherein said first optical fiber section comprises an optical fiberhaving transmission modes matching those of an erbium-doped opticalfiber section.
 6. The integrated input module of claim 5 wherein saidfirst optical fiber section comprise HI-980 optical fiber.
 7. Theintegrated input module of claim 6 wherein said end of said firstoptical fiber section comprises a mode-expanded subsection and a fibertransition subsection.
 8. The integrated input module of claim 7 whereinsaid mode-expanded subsection comprises SMF-28 optical fiber.